1. Field of the Invention
This invention relates to a composition for a display device, and more particularly to a composition of sealing glass for bonding the upper and lower substrates of a flat panel display device.
2. Description of the Prior Art
Recently, there have been actively developed flat panel display devices such as a liquid crystal display(LCD), a field emission display(FED), a plasma display panel(PDP) and so on. In the flat panel display device, the PDP has advantages in that it provides ease of manufacture of a large-scale screen due to its simple structure, and that it has a light view angle more than 160.degree. and characteristics of being thin and light weight. The PDP exploits a gas discharge phenomenon to display a picture by radiating a fluorescent material with a vacuum ultraviolet ray generated during a gas discharge. A typical structure of the PDP will be described with reference to FIG. 1 below.
FIG. 1 shows a structure of a discharge cell arranged in a matrix pattern in the conventional PDP. The PDP discharge cell includes an upper plate 28 having a sustaining electrode pair 12A and 12B, an upper dielectric layer 14 and a protective film 16 that are sequentially formed on an upper substrate 10, and a lower plate 30 having an address electrode 20, a lower dielectric layer 22, a barrier rib 24 and a fluorescent material layer 26 that are sequentially formed on a lower substrate 30. The upper substrate 10 is spaced in parallel from the lower substrate 18 by the barrier rib 24. The sustaining electrode pair included in the upper plate 28 consists of a scanning/sustaining electrode 12A and a sustaining electrode 12B. The scanning/sustaining electrode 12A is responsible for applying a scanning signal for an address discharge and a sustaining signal for a sustained discharge, etc. On the other hand, the sustaining electrode 12B is responsible for applying a sustaining signal for a sustained discharge, etc. The upper dielectric layer 14 is formed on the upper substrate 10 on which the sustaining electrode pair 12A and 12B is provided. The protective film 16 is coated on the surface of the upper dielectric layer 14. A MgO film is usually used as the protective film 16. The protective film 16 protects the upper dielectric layer 14 from the sputtering phenomenon of plasma articles to prolong the life of the PDP and improve an emission efficiency of secondary electrons. Also, the protective film 16 reduces a variation in the discharge characteristic of a refractory metal due to a contamination of oxide. The address electrode 20 included in the lower plate 30 is formed on the lower substrate 18 in such a manner to be crossed with the sustaining electrode pair 12A and 12B. The address electrode 20 serves to apply a data signal for the address discharge. The lower dielectric layer 22 is formed on the lower substrate 18 on which the address electrode 20 is provided. The barrier rib 24 is arranged in parallel to the address electrode 20 on the lower dielectric layer 22. The barrier rib 24 serves to provide a discharge space at the inner side of the discharge cell so as to shield electrical and optical interference between the adjacent discharge cells. Also, the barrier rib 24 serves to support the upper substrate 10 and the lower substrate 18. The fluorescent material layer 26 is coated on the surfaces of the lower dielectric layer 22 and the barrier rib 24 to generate a red, green, or blue visible ray. Further, an inactive gas for the gas discharge is sealed into the discharge space. The PDP discharge cell having a structure as described above maintains a discharge by a surface discharge between the sustaining electrode pair 12A and 12B after being selected by an opposite discharge between the address electrode 20 and the scanning/sustaining electrode 12A. In the PDP discharge cell, the fluorescent material 26 is radiated by an ultraviolet ray generated during the sustained discharge, thereby emitting a visible light to the outer side of the discharge cell. As a result, the PDP having discharge cells display a picture.
FIG. 2 explains a process of sealing the upper plate 28 and the lower plate 30 of the PDP shown in FIG. 1. Referring to FIG. 2, there are separately provided the upper plate 28 in which the sustaining electrode pair 12A and 12B, the upper dielectric layer 14 and the protective film 18 are sequentially disposed on the upper substrate 10, and the lower plate 30 in which the address electrode 20, the lower dielectric layer 22, the barrier rib 24 and the fluorescent material layer 26 are sequentially disposed on the lower substrate 13. Next, a PDP device is completed by sealing the upper plate 28 and the lower plate 30 after coating a sealing glass made in a paste state at the edge portion of the upper substrate 10, in which the sustaining electrode pair 12A and 12B, the upper dielectric layer 14 and the protective film 18 are not formed, in such a manner to have a width of about 1 Cm and a height of about 200 .mu.m. In this case, a glass of PbO--B.sub.2 O.sub.3 --ZnO group having a compositions as indicated in the following Table 1 is used as the sealing glass.
TABLE 1 COMPO- NENT PbO B.sub.2 O.sub.3 ZnO SiO.sub.2 Al.sub.2 O.sub.3 Na.sub.2 O Li.sub.2 O WEIGHT % 75-82 6-12 7-14 1-3 0-3 0.3-0.5 0.1-0.2
FIG. 3 is a flow chart for explaining a formation method of a sealing glass making use of a glass of PbO--B.sub.2 O.sub.3 --ZnO group step by step. Referring to FIG. 3, in step S1, a glass of PbO--B.sub.2 O.sub.3 --ZnO group having compositions as indicated in Table 1 are prepared into a powder having a particle size of about 3 to 5 .mu.p. Next, in step S2, the PbO--B.sub.2 O.sub.3 --ZnO group glass powder is mixed with an organic vehicle, thereby making a paste state. In this case, a viscosity of the paste of about 100,000 cps is a proper value. In step S3, a sealing paste is coated on the edge portion of the upper substrate 10 by exploiting the screen printing technique. Subsequently, in step S4, a sealing of the upper and lower plates 28 and 30 is completed by matching the upper plate 28 and the lower plate 30 coated with the sealing paste and then calcining the same. In this case, a sealing of the upper and lower plates 28 and 30 is terminated by sintering and crystallizing the upper and lower plates 28 and 30 matched by the sealing paste during about 20 to 30 minutes at a temperature of about 450.degree. C. under the atmosphere condition using a resistance heating furnace and cooling the same so as to form a sealing glass 32. At this time, a thermal expansion coefficient of the PbO--B.sub.2 O.sub.3 --ZnO group glass is more than 100.times.10.sup.-7.degree. C. However, if the PbO--B.sub.2 O.sub.3 --ZnO group glass is calcined, then a crystal structure of ZnB.sub.2 O.sub.4 and ZnO--2SiO.sub.2 is produced. Accordingly, a coefficient of the PbO--B.sub.2 O.sub.3 --ZnO group glass is reduced to 85 to 90.times.10.sup.-7.degree. C., and a color of the sealing glass is changed into black.
However, after the final sealing, a slight crack is produced at the sealing glass due to a stress generated from the electrodes 12A, 12B and 20 defined on the upper and lower substrates 10 and 18, the upper and lower dielectric layer 22 and the fluorescent material layer 26, etc. in the course of performing a thermal treatment for sintering the PbO--B.sub.2 O.sub.3 --ZnO group glass. Also, since a sintering temperature of the PbO--B.sub.2 O.sub.3 --ZnO group glass has a relatively low value of less then 450.degree. C., many air holes remain in the interior of the paste to deteriorate an airtightness of the PDP device. Accordingly, external air enters the internal discharge space of the PDP device to thereby deteriorate a discharge characteristic. Moreover, the PbO--B.sub.2 O.sub.3 --ZnO group glass contains above 70 weight % of toxic PbO oxide causing serious environment and work performance problems.